feat: add pytest pre-commit hook and tests scaffold (issue #162)

- pyproject.toml with pytest>=8 in [dependency-groups.dev] so
  uv run --group dev pytest resolves without a global install
- tests/__init__.py and tests/conftest.py skeleton referencing issue #162
- pre-commit local hook runs pytest tests/ on Python file changes;
  exit code 5 (no tests collected) is treated as success so commits
  are not blocked while the test suite is being built out

.pre-commit-config.yaml

@@ -31,6 +31,15 @@ repos:
             "-s",
           ]
 
+  - repo: local
+    hooks:
+      - id: pytest
+        name: Run unit tests
+        entry: bash -c 'uv run --group dev pytest tests/ -q; rc=$?; [ $rc -eq 5 ] && exit 0 || exit $rc'
+        language: system
+        types: [python]
+        pass_filenames: false
+
   - repo: https://github.com/python-jsonschema/check-jsonschema
     rev: 0.37.2
     hooks: